1. Field of the Invention
The present invention relates to a method and system for controlling auto-ignition in an internal combustion engine.
2. Description of the Background Art
JP-A 10-266878 discloses a method of controlling auto-ignition in an internal combustion engine over an engine operating range including a light load region and a middle load region. According to this method, the closing timing of an exhaust valve is set increasingly before a top dead center (TDC) during the exhaust stroke with the decreasing engine loads at the rate of a fixed change in the closing timing per a unit change in the decreasing engine loads. The opening timing of an intake valve is set increasingly after the TDC with the decreasing engine loads at the rate of the fixed change.
This manner of valve timing control is satisfactory over limited range of engine loads to provide thermal energy to sustain cylinder temperature high enough for auto-ignition. However, a need remains to develop valve-timing strategies to provide cylinder temperature high enough for auto-ignition over a wider range of engine loads. At light load region of an engine operating range for auto-ignited combustion, the ignitability of auto-ignition drops and it is found to be difficult to elevate cylinder temperature to a sufficiently high level. This stems from the fact that at light load region, substantial portion of thermal energy of the retained exhaust gas is lost through the cylinder wall so that a gain in thermal energy against a unit increase in exhaust gas retaining duration is small. At heavy load region of such engine operating range for auto-ignited combustion, the ignitability of auto-ignition is enhanced so that it is no longer necessary to retain substantial amount of retained exhaust gas by setting the exhaust gas retaining duration.
It is an object of the present invention to provide a method and system for controlling auto-ignition in an internal combustion engine to reliably sustain auto-ignition over a wider engine operating range including such light load region where the ignitability of auto-ignition is likely to drop.
According to one aspect of the present invention, there is provided a method for controlling an auto-ignition in an internal combustion engine including a cylinder with an inlet control device and an outlet control device, comprising:
setting the closing timing of the outlet control device increasingly before a top dead center (TDC) during the exhaust stroke of each engine cycle with decreasing engine loads at the rate of a change in the closing timing per a unit change in the decreasing engine loads, which change in the closing timing increases with the decreasing engine loads; and
setting the opening timing of the inlet control device after the TDC.